This invention relates to a fusing system in an image forming system for fixing a toned image to a support member. More particularly, this invention relates to an external heating source for providing radiant thermal energy to a fuser member surface.
In a typical image forming system, a light image of an original to be copied is recorded in the form of an electrostatic latent image upon a photosensitive imaging member. The latent image is subsequently rendered visible by the application of electroscopic thermoplastic resin particles, which are commonly referred to as toner. The visible toned image is then in a loose powdered form and can be easily disturbed or destroyed. The toned image is usually fixed or fused upon a support sheet such as plain paper.
The use of thermal energy for fixing developed toned images onto a support member is well known. To fuse electroscopic toner material onto a support surface permanently by heat, it is usually necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky. This heating causes the toner to flow to some extent into the fibers or pores of the support member. Thereafter, as the toner material cools, solidification of the toner material causes it to be firmly bonded to the support.
Typically, toner particles are fused to a print substrate by heating to a temperature of between about 90xc2x0 C. to about 160xc2x0 C. or higher, depending upon the softening range of the particular resin used in the toner. It is not desirable, however, to raise the temperature of the substrate substantially higher than about 200xc2x0 C. because of the tendency of the substrate to discolor at such elevated temperatures, particularly when the substrate is paper.
Several approaches to thermal fusing of electroscopic toned images have been described. These methods include providing the application of heat and pressure substantially concurrently by various means, a roll pair maintained in pressure contact, a belt member in pressure contact with a roll, a belt member in pressure contact with a heater, and the like. Heat may be applied by heating one or both of the rolls, plate members, or belt members. The fusing of the toner particles generally takes place when the proper combination of heat, pressure and contact time is provided. The balancing of these parameters to bring about the fusing of the toner particles is well known in the art, and they can be adjusted to suit particular machines, process conditions, and printing substrates.
During operation of a fusing system in which heat is applied to cause thermal fusing of the toner particles onto a support, both the toner image and the support are passed through a nip formed between the roll pair, or plate, and/or belt members. The concurrent transfer of heat and the application of pressure in the nip effects the fusing of the toner image onto the support. It is important in the fusing process that no offset of the toner particles from the support to the fuser member takes place during normal operations. Toner particles offset onto the fuser member may subsequently transfer to other parts of the machine or onto the support in subsequent copying cycles, thus, increasing the background or interfering with the material being copied. The so called xe2x80x9chot offsetxe2x80x9d occurs when the temperature of the toner is raised to a point where the toner particles liquefy and a splitting of the molten toner takes place during the fusing operation with a portion remaining on the fuser member.
The hot offset temperature or degradation of the hot offset temperature is a measure of the release property of the fuser roll, and accordingly it is desired to provide a fusing surface that has a low surface energy to provide the necessary release. To ensure and maintain good release properties of the fuser roll, it has become customary to apply release agents to the fuser members to ensure that the toner is completely released from the fuser roll during the fusing operation. Typically, these materials are applied as thin films of, for example, silicone oils to prevent toner offset.
A feature common to most of the prior art fuser members is that the source of the heat energy for the fusing operation is generally in the form of a quartz lamp positioned in the core of a fuser member. In such a configuration, the heat must be conducted from the core of the fuser member, through the various layers of materials comprising the fuser member, to the surface of the fuser member for fusing the toned image to the copy substrate. To obtain the proper higher fusing temperature needed for fusing at the surface of such a fusing member, the temperatures at the various layers or points within the fuser member must be substantially higher. Since heat must be transmitted from the source in the core of the fuser member to its surface, it takes an appreciable amount of time before the surface of the fusing member is warmed up to the fusing temperature and thus ready for operations. This delay in readiness of the machine to fuse toned images, or the warm-up time, is accentuated by the fact that such fuser members are generally made of elastomeric or other polymeric materials, which are generally poor conductors of heat.
To solve some of the above problems that occur with internally heated fuser members, an external heating source has been used. This external heating source is associated with the fusing member so as to provide heat to the surface of the fusing member.
U.S. Pat. No. 4,071,735 discloses an externally heated roll fuser, in which the heating element heats the fuser roll at the same time preheats the toned image to be fused. The fuser roll of this patent is made of a metallic core with a layer of heat insulating silicone rubber thereon.
U.S. Pat. No. 6,061,545 discloses an external heating element for a fusing system having a heat source, a substrate and an outer fluoropolymer layer with a fluoropolymer and a conductive filler.
Although external heating elements provide benefits to image fusing, problems with the use of external heating sources persist. The external heating sources in the prior art are primarily high temperature (2000xc2x0 C.) radiant heat line sources (lamps) providing the necessary heat energy to a targeted location on the fuser member surface. The cost, heater element dimensions and the nature of the heat source restrict the number of components and dictate that heat must be applied to a limited portion of a fuser member surface. Consequently, this necessitates high heat fluxed and elevated temperatures to transfer heat energy to a fuser member in the limited time that is available as a fuser member rotates past an external heater. A particular disadvantage to the external heating elements for fuser members of the prior art is non-uniform heating of a fuser roll surface. While one location of the fuser member surface receives a sufficient amount of heat, other locations may receive too much or too little heat. Non-uniform heating of a fuser roll surface results in inconsistent heating of the toner particles, which then results in irregularities and inconsistencies in the final image. This arrangement is also susceptible to paper or substrate fires in the event of a fuser wrap, which occurs when paper or substrate adheres to the fuser member surface and is carried to the location where heat is applied to the surface. The temperature of the heating element is sufficient to ignite the paper or substrate under either direct contact or prolonged exposure. An additional disadvantage of prior external heating elements is difficulty in controlling the temperature on the surface of the fuser member. An unsuitable temperature causes incomplete fusion of the image to the print substrate, discoloration of the print substrate and other undesirable effects.
The present invention provides an external heating source for a fuser member used in an image forming system. According to one practice of the invention, the surface of the fuser member is heated with thermal radiation generated by an external heating source located near the surface of the fuser member. Heat is generated by a resistive radiator on the surface of the external heating source, comprised of a ceramic-containing material having a high resistance, such as a transition metal oxide.
The present invention provides an external heating source for a fuser member, wherein high quality prints and/or copies are produced. Particularly, the invention provides an external heating source demonstrating improved temperature control and uniformity in heating a fuser member. The external heating source of the present invention maintains an ideal temperature throughout the surface of the fuser member for extended periods of time. As a result, proper fusion of a toned image to a print substrate is ensured, and a high-quality final image is consistently printed.
According to one aspect, the present invention provides a fusing system for fusing a toned image to a support sheet in an image forming system. The fusing system includes a fuser member and an external heating source for supplying thermal radiation to heat a surface of the fuser member. The external heating source includes a substrate and a ceramic-containing coating applied to an outer surface of the substrate.
According to another aspect, the present invention provides a method of heating a surface of a fuser member in an image forming system. The method comprises the steps of providing an external heating source comprising a substrate and a ceramic-containing coating, and supplying radiant thermal energy generated by the heating source to the fuser member.
According to still another aspect, the present invention provides a fusing system including a fuser member and an external heating source wherein the external heating source comprises a plurality of ceramic-containing coating strips formed on a substrate. In an alternate embodiment, the external heating source further comprises at least one temperature sensor, such as a thermistor, coupled to the ceramic-containing coating to control temperature on the surface of the fuser member.